This application is based upon provisional patent application Ser. No. 60/573,911 filed May 24, 2004.
The invention deals generally with a mold for manufacturing information recording discs such as compact discs, CDs, and DVDs. In order to mold these discs a thin plate containing a mirror image of digital information is held inside the mold cavity. This plate is typically referred to as a “stamper plate” or simply a “stamper”, is usually made of nickel, and the digital information is represented as a series of pits and grooves. The stamper generally has an outer diameter of about 138 mm., an inner diameter, at a central hole, of 20.0 to 37.4 mm., and a thickness of approximately 0.32 mm.
The material for the disc, which is molded in contact with the stamper to create a duplicate of a master, is generally a polycarbonate plastic. The stamper is held centered within the mold cavity at its inner diameter and held in the mold by vacuum, magnetic, or mechanical means, or a combination of these, acting at the stamper's inner diameter, outer diameter, or along the surface of the underside of the stamper. These mold components are referred to as “stamper holders”.
One typical mechanical means used by these stamper holders to hold the stamper inner diameter is a low profile lip, but the impression of this lip is then molded into the disc. This lip impression may cause problems for the silk screen printing that occurs as a secondary operation after molding by causing printing distortions or damage to the silk screen used in the printing.
To alleviate this problem stamper holders have been produced that eliminate the mechanical lip feature. Such designs then permit disc manufacturers to print closer to the inner diameter of the molded disc, thereby creating an aesthetic improvement and a market advantage.
This has been done by the use of temperature resistant magnets incorporated into the stamper holder to hold the stamper in place. Typically this involves a centering feature that is approximately 20 mm. in diameter and is raised approximately 0.320 mm. above the planar surface of the stamper holder that includes embedded magnets. The stampers used in such a system are manufactured with a 20 mm. inner diameter so that they will center properly on such a stamper holder.
The magnets embedded in the stamper holder surface are typically rod shaped magnets, and they are placed in a circular pattern around the raised centering feature. These magnets are usually formed from a temperature resistant rare earth material such as samarium cobalt, neodymium iron boron, or the like, and they are typically held within the stamper holder with a temperature resistant resin.
However, since these magnets are exposed directly to the stamper, several problems can occur during high production molding processes. Exposed magnets tend to chip or break thereby releasing contamination beneath the stamper, and this causes molding failures and damage to the stamper and other mold components. Another failure mode is caused by poor adhesion. This allows the magnets to lift out of their recesses during use and causes molding failures and precipitates magnet chipping and breaking.
Wear is another problem. Thin wear resistant films such as those deposited by physical vapor deposition, chemical vapor deposition, plasma assisted chemical vapor deposition, thermo-reactive diffusion, and carborizing and nitro-carborizing are applied to stamper holders to improve wear resistance, but they do not adhere well to rare earth materials. When the coating flakes off of the magnets there is no protection for this most fragile part of the assembly. Furthermore, the exposed surfaces of the magnets make it difficult to achieve a good surface finish on the stamper holder surface with which the stamper is in contact. This poor surface also causes wear, especially on the stamper.
Finally, the rod magnets in a circular pattern may not have sufficient magnetic strength. This causes distortion of the stamper and disc around the inner diameter which detracts from the aesthetics, and may even cause discs to be rejected on quality criteria.
It would be very beneficial to have available a magnetic stamper holder that did not have these problems, so that the ease of use of the magnetic stamper holder was not negated by poor wear characteristics and contamination from the magnets chipping and breaking.